This invention relates generally to the field of memory devices, and more particularly to a pipelined packet-oriented synchronous DRAM interface.
Conventional memory devices have a standard interface consisting of separate address, data and control pins. For example, one version of a synchronous dynamic random access memory (SDRAM) has twelve address pins, two multiplexed address and control pins, seven control pins and sixteen data pins. This approach offers a great deal of flexibility since computer systems vary greatly in their memory requirements. In particular, the bandwidth of memory systems using SDRAMs can easily be increased by adding another SDRAM in parallel to the existing SDRAMs, thereby increasing the width of the memory bus.
The tradeoff for this flexibility is, however, an increase in layout space which leads to an increase in manufacturing cost. Separate traces need to be run for each pin of each SDRAM. Therefore, it is sometimes cost prohibitive to use SDRAMS for wide memory systems.
One approach to lower the cost of expanding memory is to use memory devices which multiplex address, control and data information on the same pins. For example, some memory devices have a set of generic interface pins which connect to a high-speed, synchronous bus. Communication over the bus is accomplished by a series of packets which conform to a predefined packet protocol. Usually the packet protocol is fairly sophisticated and has a complete command set. For example, DRAMS conforming to the RAMBUS(trademark) interface communicate using a protocol in which each packet consists of six bytes transmitted sequentially over a high-speed bus known as a xe2x80x9cChannel.xe2x80x9d In this manner, the packets encapsulate all address, control and data information.
Because of the efficient use of generic interface pins, a packet protocol reduces the required number of pins to approximately 30. However, this has the disadvantage of decreasing effective data bandwidth, because only a portion of the total bus bandwidth is available for data (the rest of the bandwidth is reserved for address and control information).
Another method for reducing the cost associated with increasing total memory bandwidth, without decreasing effective data bandwidth, is to provide a second high-speed bus specifically for communicating data. In this approach, address and control information is communicated over a unidirectional high-speed address/control bus while data is communicated over a bidirectional high-speed data bus. Both communications conform to a predefined packet protocol. This approach has the benefits of reducing the total pin count (although not as much as the RAMBUS(trademark) protocol described above), yet has the added benefit that only the data bus needs to be duplicated when the width of the memory system is increased.
Both approaches described above offer advantages over traditional memory architectures in terms of increased data retrieval bandwidth. It is difficult, however, to implement systems having both fine granularity and large memory depth using such devices. What is needed is a memory architecture which supports increased bandwidth, fine granularity, and large memory arrays.
As explained in detail below, an improved memory system is provided having a unidirectional command and address bus coupled to a memory controller, the memory controller communicating commands and addresses to the command and address bus. A bidirectional data bus is also coupled to the memory controller, the memory controller communicating data information to the bidirectional data bus for a write operation and receiving the data information from the bidirectional data bus during a read operation. The memory system further includes a plurality of memory devices, a buffer register connected between the command and address bus and the plurality of memory devices, the buffer register receiving and latching the commands and addresses from the command and address bus and driving the commands and addresses to the plurality of memory devices, and a data register connected between the plurality of memory devices and the bidirectional data bus, the data register receiving and latching the data information from the bidirectional data bus and driving the data information to the plurality of memory devices for a write operation, the data register receiving and latching the data information from the plurality of memory devices and driving the data information to the bidirectional data bus for a read operation.
In another embodiment of the invention, the memory system has a unidirectional command and address bus coupled to a memory controller, the memory controller communicating commands and addresses to the command and address bus, and a bidirectional data bus coupled to the memory controller, the memory controller communicating data information to the bidirectional data bus for a write operation and receiving the data information from the bidirectional data bus during a read operation. The memory system further includes a plurality of pipelined memory subsystems, each memory subsystem having a plurality of memory devices, a buffer register connected between the command and address bus and the plurality of memory devices, the buffer register receiving and latching the commands and addresses from the command and address bus and driving the commands and addresses to the plurality of memory devices, and a data register connected between the plurality of memory devices and the bidirectional data bus, the data register receiving and latching the data information from the bidirectional data bus and driving the data information to the plurality of memory devices for a write operation the data register receiving and latching the data information from the plurality of memory devices and driving the data information to the bidirectional data bus for a read operation.
According to another feature of the invention, the memory system includes a unidirectional command and address bus coupled to a memory controller, the memory controller communicating commands and addresses to the command and address bus, and a bidirectional data bus coupled to the memory controller, the memory controller communicating data information to the bidirectional data bus for a write operation and receiving the data information from the bidirectional data bus during a read operation. The memory system further includes a memory module including a pipelined memory subsystem. The pipelined memory subsystem includes: a) a plurality of memory devices, b) a buffer register connected between the command and address bus and the plurality of memory devices, the buffer register receiving and latching the commands and addresses from the command and address bus and driving the commands and addresses to the plurality of memory devices, and c) a data register connected between the plurality of memory devices and the bidirectional data bus, the data register receiving and latching the data information from the bidirectional data bus and driving the data information to the plurality of memory devices for a write operation, the data register receiving and latching the data information from the plurality of memory devices and driving the data information to the bidirectional data bus for a read operation. The memory system also has a socket adapted to receive the memory module and to couple the pipelined memory subsystem of the memory module to the unidirectional command and address bus and to the bidirectional data bus.
In order to solve the problems in the prior art, we have provided a method for storing data in a pipelined memory system. The method includes the steps of communicating commands and addresses to a unidirectional command and address bus, communicating data information to a bidirectional data bus, latching the commands and addresses in a plurality of buffer registers, latching the data in a plurality of data registers, driving the latched commands and addresses to a plurality of memory devices having addressable storage, driving the latched data to the plurality of memory devices, and storing the data in the addressable storage of one of the plurality of memory devices.
These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.